Calvin-Benson cycle cells

The overall effect is to transport C02 from the mesophyll cells into the bundle sheath cells along with two reducing equivalents, which appear as NADPH following the action of the malic enzyme. The C02, the NADPH, and additional NADPH generated in the chloroplasts of the bundle sheath cells are then used in the Calvin-Benson cycle reactions to synthesize 3-phospho-glycerate and other materials. Of the C02 used in the bundle sheath cells, it is estimated that 85% comes via the C4 cycle and only 15% enters by direct diffusion. The advantage to the cell is a higher C02 tension, less competition with 02, and a marked reduction in photorespiration. 

As a result, oxaloacetate (OAA, C4-compound) is formed unlike the case of the Calvin-Benson cycle in which 3-phosphoglycerate (C3-compound) is formed. The pathway in the fixation of carbon dioxide by the catalysis of PEP-carboxylase is observed in sugar cane, corn, etc., and is called the Hatch-Slack pathway (Hatch et al., 1967). The plants having the Hatch-Slack pathway have chloroplasts both in mesophyll cells and in vascular bundle sheath cells, and the Hatch-Slack pathway occurs in the mesophyll cells. Oxaloacetate formed by the fixation of carbon dioxide in the mesophyll cells is reduced to malate. Malate thus formed moves to the vascular bundle sheath cells and releases carbon dioxide there. Carbon dioxide released is fixed by the catalysis of Rubisco, and the organic compounds are formed through the Calvin-Benson cycle. (Fig. 6.3). 

The C4 cycle can be viewed as an ATP-dependent C02 pump that delivers C02 from the mesophyll cells to the bundle-sheath cells, thereby suppressing photorespiration (Hatch and Osmond, 1976). The development of the C4 syndrome has resulted in considerable modifications of inter- and intracellular transport processes. Perhaps the most striking development with regard to the formation of assimilates is that sucrose and starch formation are not only compartmented within cells, but in C4 plants also may be largely compartmented between mesophyll and bundle-sheath cells. This has been achieved together with a profound alteration of the Benson-Calvin cycle function, in that 3PGA reduction is shared between the bundle-sheath and mesophyll chloroplasts in all the C4 subtypes. Moreover, since C4 plants are polyphyletic in origin, several different metabolic and structural answers have arisen in response to the same problem of how to concentrate C02. C4 plants have three distinct mechanisms based on decarboxylation by NADP+-malic enzyme, by NAD+-malic enzyme, or by phosphoenolpy-ruvate (PEP) carboxykinase in the bundle-sheath (Hatch and Osmond, 1976). 

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